Interpretive Summary: Stocker cattle are a key part of agriculture in the southern Great Plains (SGP), where they graze different forages to gain weight before entering feedlots for finishing. Producers in the region need sources of high quality forage during late summer to early fall (July to October). However, this time of year tends to regularly experience drought, so any annual forage grown for grazing must be capable of producing biomass on limited amounts of water. We applied a study to explore the function of pigeon pea as late-summer forage for stocker cattle. Water use and water use efficiency of two pigeon pea cultivars (Georgia 2 and Minnesota 8) were compared with and without grazing. A set of one-acre pastures were planted to one of the cultivars in June of 2008 to 2010, and allowed to grow until late August. Crossbred stocker cattle were then assigned to the paddocks and allowed to graze until the end of the growing season (October). The amount of soil water used in the upper 30 inches of soil and water use efficiency of the two cultivars, with and without grazing, was described at flowering and end of grazing periods. Results showed most of the noted effects on available soil water, water use, or water use efficiency during the study was more related to growing conditions during individual years than cultivar or grazing treatment. Amount and timing of precipitation, and air temperature, should be the main criteria in choosing to use pigeon pea as summer forage.

Technical Abstract:
Water use by the warm-season annual pulse pigeon pea must be described to effectively use this legume as forage to support late-summer grazing by stocker cattle in the southern Great Plains (SGP). This study was conducted in central Oklahoma during 2008 to 2010 to quantify water and water use efficiency by two pigeon pea cultivars (GA2 and MN8), with and without grazing. Paddocks (n=6, 0.41 ha; n=3 cultivar-1) were randomly assigned cultivar treatments, sprayed pre-plant with glyphosphate (1.54 kg ai ha-1 y-1), fertilized with 26 kg ha-1 P (in 130 kg ha-1 of 18-46-0 dry fertilizer), and planted with inoculated seeds (60-cm row spacing; 25 kg ha-1 rate) in late May to early June. Enclosures (9 m-2) were established to measure biomass, and crossbred stocker cattle (n=3) were assigned to each paddock for grazing (August through October). Available water in the upper 75 cm of soil was quantified by neutron density gage at planting, flowering and end of grazing period, to calculate water use and water use efficiency (WUE). Grazing main and interaction effects on available soil water were not significant (0.13< p<0.50) or water use (0.10<p<0.62), while cultivar x year x time of season interactions were (p=0.02). The lowest water use occurred during the driest year (2009), with the overall lowest rate noted at flowering. Main effects of year and time of season on WUE were significant (p<0.01). Results indicated annual and inter-annual climatic conditions had greater effects on water use by pigeon pea than grazing